The measurement of various body constituents by the use of radioimmunoassay techniques has achieved widespread acceptance in recent years. Exemplary of substances which can be measured by radioimmunoassay using currently available commercial kits are ACTH (adrenocorticotropin), aldosterone, angiotensin I, angiotensin II, barbiturates, cyclic AMP, cyclic GMP, digoxin, folic acid, FSH (follicle stimulating hormone), gastrin, HAA (hepatitis associated antigen), HGH (human growth hormone), insulin, TSH (thyroid stimulating hormone), thyroxine (T4), triiodothyronine (T3), and vitamin B12.
Radioimmunoassay tests require a specific antibody, a radioisotope-labeled (hereinafter referred to as "radiolabeled") antigen, a pure sample of the antigen to be measured to serve as a reference standard, and means for the separation of free antigen from antibody-bound antigen. Radioimmunoassays follow the basic principle of saturation analysis, i.e., competition between labeled and unlabeled antigen for a fixed number of antibody binding sites. The term "antigen", as used in the field of radioimmunoassays, may cover substances of limited antigenicity. In those cases where the substance to be measured is of limited antigenicity, the substance can be coupled with an agent which will increase its antigenicity.
When radiolabeled antigen, unlabeled antigen, and antibody are brought together, the amount of radiolabeled antigen bound to antibody and the amount of radiolabeled antigen remaining unbound (free) has a direct relationship to the amount of unlabeled antigen present when a given amount of antibody is present. Thus, by using a constant amount of antibody and radiolabeled antigen, and using known concentrations of unlabeled antigen, a standard (calibration) curve can be plotted showing antigen concentration versus the amount of radiolabeled antigen bound or versus radiolabeled antigen unbound, or versus a ratio of the two measurements. The concentration of antigen in an unknown sample can be read from the standard curve by determining the amount of bound or free radiolabeled antigen (or ratio of the two measurements) resulting when the unknown sample is mixed with the amount of radiolabeled antigen and antibody used to prepare the curve.
One body constituent for which no direct radioimmunoassay procedure is available is renin. Renin is an enzyme that is synthesized, stored and released from granules contained in the juxtaglomerular apparatus of the kidney. It is generally accepted that the major physiological role of renin is to act as the primary stimulus for maintenance of body sodium balance. Plasma renin activity is increased when there is a decreased renal perfusion pressure, and when there is a decreased delivery of sodium and water to the distal tubule. Renin acts on substrate, angiotensinogen, an .alpha.-2-globulin produced by the liver, to form a decapeptide, angiotensin I. Angiotensin I is biologically inactive, and is converted to the biologically active octapeptide, angiotensin II, in the pulmonary circulation. Angiotensin II acts as a highly potent vasopressor, and stimulates the adrenal gland to produce aldosterone. Aldosterone promotes the reabsorption of sodium by the distal tubule, and when secreted in excessive amounts, results in hypertension.
Plasma renin assays are used as an adjunct in determining whether hypertension is due to primary aldosteronism or to renal vascular disease. The hypertensive patient that has plasma renin levels suppressed below normal, and has a high aldosterone secretion rate will usually be found to have adrenal hyperplasia or an adenoma of the zona glomerulosa of the adrenal gland. In patients with renal artery stenosis, the renal venous blood from the ischemic kidney may be found to have higher plasma renin levels than renal venous blood from the unaffected kidney.
As described above the determination of plasma renin activity is a valuable tool in a physician's diagnosis and differentiation of hypertensive disorders. Because purified renin is not available, the direct radioimmunoassay of renin is not possible. However, if it is assumed that the angiotensin I generated by a plasma substrate is proportional to the plasma renin activity of that substrate, it is possible to determine the plasma renin activity of a substrate by using radioimmunoassay procedures to determine the amount of angiotensin I generated by that substrate.